The feasibility and adaptability of the basic chemistry of zeolite catalyzed conversion of oxygenates and olefins to produce higher hydrocarbons has been the subject of much inventive research activity. Recent developments in zeolite catalyzed hydrocarbon conversion processes have created interest in using olefinic feedstocks for producing C.sub.5 + gasoline, diesel fuel, etc. In addition to the basic work derived from ZSM-5 type zeolite catalyst, a number of discoveries have contributed to the development of a new industrial process, known as Mobil Olefins to Gasoline/Distillate ("MOGD"). This process has significance as a safe, environmentally acceptable technique for utilizing feedstocks that contain lower olefins, especially C.sub.2 -C.sub.5 alkenes. Conversion of lower olefins to gasoline and/or distillate products is disclosed in U.S. Pat. No. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins, are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of a ZSM-5 type zeolite. In a related manner, U.S. Pat. No. 4,150,062, 4,211,640 and 4,227,992 (Garwood, et al) disclose processes for converting olefins to gasoline and/or distillate components. The foregoing disclosures are incorporated herein by reference.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using a medium pore shape selective acid crystalline zeolite, such as ZSM-5 type catalyst, process conditions can be varied to favor the formation of either gasoline or distillate range products. At moderate temperature and relatively high pressure, the conversion conditions favor aliphatic distillate range product having a normal boiling point of at least 165.degree. C. (330.degree. F.). Lower olefinic feedstocks containing C.sub.2 -C.sub.8 alkenes may be converted. The distillate product produced from olefins oligomerization represents an advantageous source for diesel fuel and the like; however, the oligomerization product contains olefinic unsaturation which must be hydrogenated to produce paraffins having a cetane value compatible with the intended product use. Rather than construct an independent hydrotreating operation for hydrogenating the MOGD product, if technically feasible, the use of existing hydrotreating operations is to be preferred. One such commonly available operation found in the refinery setting is catalytic hydrodesulfurization.
Catalytic hydrodesulfurization, or CHD, is a well-known process used to remove sulfur from sulfur-bearing fuel oils by hydrogenation to produce hydrogen sulfide. Typically, further hydroconversion of the feed is not realized in the CHD operation. Hydrocarbon feed materials which may be successfully desulfurized in the process include straight run hydrocarbons or hydrocarbon materials of cracking operations. Generally, the process is conducted at elevated temperatures between 260.degree. C. and 400.degree. C. and pressures between 3500 kPa and 21000 kPa. The process can use a wide range of hydrogenation catalysts including catalysts incorporating chromium, molybdenum, nickel, platinum, tungsten and the like, either alone or in mixtures, on supports such as silica or alumina.
It has been discovered that feeding a stream containing a significant quantity of olefinic materials, such as the product of an MOGD process, to an existing CHD unit in order to combine hydrodesulfurization of the usual feed to the CHD unit with hydrogenation of the MOGD product results in an excessive temperature rise in the unit which, in turn, results in a reduction in the CHD cycle and increase in the frequency of catalyst regeneration. The effect renders the process so combined uneconomic. The cause of the high temperature rise is the high exotherm of the olefin hydrogenation reaction. Accordingly, workers in the field have sought ways to moderate or otherwise manage this high exotherm so that the MOGD product may be combined with CHD feed to permit utilization of the CHD operation for the hydrotreating of olefinic MOGD product to produce a hydrogenated product having higher cetane number.
Accordingly, it is an object of the present invention to provide a process for the integration of MOGD product hydrotreating with CHD feed hydrotreating.
Another object of the present invention is to provide a process to convert the product of olefins oligomerization to distillate fuel having higher cetane number.
Yet another object of the present invention is to provide a unique reactor means effective for the combined hydrotreating of the typical feed to a CHD operation plus feeds containing highly exothermic unsaturation, such as the product of the MOGD olefins oligomerization process.